In German Pat. No. 24 19 053 and U.S. Pat. No. 3,896,912, hydrostatic clutches are described which utilize a positive displacement gear pump to couple the input and output shafts.
In these clutches, a fluid path is established through the positive displacement gear pump which is controlled in part by a valve so that when flow along this path is blocked, one clutch operation state is defined while, when flow is permitted, the second clutch operating state effects ensured. The valve is preferably a stepless control of throttling from zero flow to a maxiumum flow position and means are provided to permit flushing of the pump with fluid when the valve is in a fully blocking position. While such clutches have been found to be highly effective because they allow coupling of the two shafts with reduced residual torque and slip, the prior art clutches of this type have been found to be effective only in one rotation sense.